Add onboarding completion endpoint and frontend integration

Commit notes

- Introduced a new API endpoint for completing user onboarding, allowing users to optionally become creators. - Updated the User model to include onboarding completion status. - Implemented onboarding route in the frontend, guiding users through the onboarding process and handling state updates. - Enhanced AuthProvider to manage onboarding state and refresh user data post-onboarding. - Added navigation logic to redirect users to the onboarding page if they haven't completed it yet, improving user flow.